Function applyQubitMeasurementAndGetProb

quest/src/api/operations.cpp:1609–1632

}

int applyQubitMeasurementAndGetProb(Qureg qureg, int target, qreal* probability) {
    validate_quregFields(qureg, __func__);
    validate_target(qureg, target, __func__);

    // we do not assume state normalisation (that is posteriori checked),
    // so we must perform two reductions; one for each outcome. We choose
    // to re-enumerate the state (potentially doubling caching costs) to
    // avoid the nuisances/race-cons of parallel adding to two scalars.
    vector<qreal> probs(2);
    probs[0] = calcProbOfQubitOutcome(qureg, target, 0); // harmlessly re-validates
    probs[1] = calcProbOfQubitOutcome(qureg, target, 1); // " "
    validate_measurementProbsAreNormalised(probs, __func__);

    // randomly choose the outcome
    int outcome = rand_getRandomSingleQubitOutcome(probs[0]);
    *probability = probs[outcome];

    // collapse to the outcome
    (qureg.isDensityMatrix)?
        localiser_densmatr_multiQubitProjector(qureg, {target}, {outcome}, *probability):
        localiser_statevec_multiQubitProjector(qureg, {target}, {outcome}, *probability);

    return outcome;
}

qreal applyForcedQubitMeasurement(Qureg qureg, int target, int outcome) {
    validate_quregFields(qureg, __func__);